Green-emitting, garnet-based phosphors in general and backlighting applications

ABSTRACT

Disclosed herein are green-emitting, garnet-based phosphors having the formula (Lu 1-a-b-c Y a Tb b A c ) 3  (Al 1-d B d ) 5 (O 1-e C e ) 12 : Ce,Eu, where A is selected from the group consisting of Mg, Sr, Ca, and Ba; B is selected from the group consisting of Ga and In; C is selected from the group consisting of F, Cl, and Br; and 0≦a≦1; 0≦b≦1; 0&lt;c≦0.5; 0≦d≦1; and 0&lt;e≦0.2. These phosphors are distinguished from anything in the art by nature of their inclusion of both an alkaline earth and a halogen. Their peak emission wavelength may lie between about 500 nm and 540 nm; in one embodiment, the phosphor (Lu,Y,A) 3 Al 5 (O,F,Cl) 12 : Eu 2+  has an emission at 540 nm. The FWHM of the emission peak lies between 80 nm and 150 nm. The present green garnet phosphors may be combined with a red-emitting, nitride-based phosphor such as CaAlSiN 3  to produce white light.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.14/023,062 Filed Sep. 10, 2013, entitled GREEN-EMITTING, GARNET-BASEDPHOSPHORS IN GENERAL AND BACKLIGHTING APPLICATIONS, by Yusong Wu et al.,now U.S. Pat. No. 9,023,242, which is a continuation of U.S. patentapplication Ser. No. 13/181,226 filed Jul. 12, 2011, entitledGREEN-EMITTING, GARNET-BASED PHOSPHORS IN GENERAL AND BACKLIGHTINGAPPLICATIONS, by Yusong Wu et al., now U.S. Pat. No. 8,529,791, whichclaims the benefit of priority of U.S. Provisional Application No.61/364,321, filed Jul. 14, 2010, entitled GREEN-EMITTING, GARNET-BASEDPHOSPHORS IN GENERAL AND BACKLIGHTING APPLICATIONS, by Yusong Wu et al.,and is a continuation-in-part of U.S. patent application Ser. No.11/975,356 filed Oct. 18, 2007, entitled NANO-YAG:CE PHOSPHORCOMPOSITIONS AND THEIR METHODS OF PREPARATION, by Dejie Tao et al., nowU.S. Pat. No. 8,133,461, which claims the benefit of priority of U.S.Provisional Application No. 60/853,382 filed Oct. 20, 2006, entitledNANO YAG:CE PHOSPHORS AND THE METHOD OF PREPARING THE SAME, by Dejie Taoet al., which applications are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present disclosure are directed in general togreen-emitting, garnet based phosphors that are applicable to a numberof different technologic areas, particularly backlighting applications.Specifically, the present disclosure is directed to lutetium-basedgarnets

2. Description of the Related Art

Embodiments of the present invention are directed to cerium-doped,garnet-based phosphors. YAG:Ce is an example of such a phosphor, andthis compound has for some time been used in commercial marketsgenerating white light from so-called “white light LEDs.” This latterterm is a misnomer, since light emitting diodes emit light of a specificmonochromatic color and not a combination of wavelengths perceived aswhite by the human eye, but the term is nonetheless entrenched in thelexicon of the lighting industry.

In comparison to other phosphor hosts, particularly those based on thesilicates, sulphates, nitridosilicates, and oxo-nitridosilicates, YAG:Cehas a relatively high absorption efficiency when excited by blue light,is stable in high temperature and humidity environments, and has a highquantum efficiency (QE>95%), all the while displaying a broad emissionspectrum.

If there is a disadvantage in using a YAG:Ce phosphor, and there is incertain applications, it is that the peak emission of this phosphor istoo long (that is to say, too deep into the red), for use as aluminescent source in, for example, a backlighting application.

An alternative to YAG:Ce is the cerium doped Lu₃Al₅O₁₂ compound(LAG:Ce), which has the same crystalline structure as YAG:Ce, a similartemperature and humidity stability as the yttrium-based compound, andlikewise for quantum efficiency. Despite these similarities, LAG:Ceexhibits a different peak emission wavelength than its YAG counterpart;in the lutetium case, this peak wavelength is at about 540 nm Thisemission wavelength is still not short enough, however, to be useful inbacklighting applications.

Thus, what is needed in the art, particularly in the fields related tobacklighting technologies, is a phosphor with a garnet structure and apeak emission wavelength shorter than that of either YAG:Ce or LAG:Ce.

SUMMARY OF THE INVENTION

Embodiments of the present disclosure are directed to green-emitting,garnet-based phosphors having the general formula(Lu_(1-a-b-c)Y_(a)Tb_(b)A_(c))₃(Al_(1-d)B_(d))₅(O_(1-e)C_(e))₁₂:Ce,Eu,where A is selected from the group consisting of Mg, Sr, Ca, and Ba; Bis selected from the group consisting of Ga and In; C is selected fromthe group consisting of F, Cl, and Br; 0≦a≦1; 0≦b≦1; 0<c≦0.5; 0≦d≦1; and0<e≦0.2. The “A” element, which may be any of the alkaline earthelements Mg, Sr, Ca, and Ba, used either solely or in combination, isvery effective in shifting wavelength to shorter values. These compoundswill be referred to in the present disclosure as “halogenated LAG-based”garnets.

In an alternative embodiment, the present green garnets may berepresented by the formula (Y,Lu,A)_(x)(Al)₅(O,F,Cl)_(12+(3/2)x);subject to the proviso that x is not equal to 3, and ranges from about2.5 to about 3.5. As in the first formula described in this section, Ais selected from the group consisting of Mg; Sr, Ca, and Ba, and rangesin content stoichiometrically from greater than equal to zero to about0.5, relative to the total amounts of yttrium and lutetium. Yttrium andlutetium are interchangeable with one another. These compounds may becollectively described in the present disclosure as “non-integerstoichiometric compounds” based on YAG and LAG.

In an alternative embodiment, the present green-emitting, garnet-basedphosphors may be described by the formula (Y,A)₃(Al,B)₅(O,C)₁₂:Ce³⁺,where A is at least one of Tb, Gd, Sm, La, Lu, Sr, Ca, and Mg, includingcombinations of those elements, wherein the amount of substitution ofthose elements for Y ranges from about 0.1 to about 100 percent in astoichiometric manner. B is at least one of Si, Ge, B, P, and Ga,including combinations, and these elements substitute for Al in amountsranging from about 0.1 to about 100 percent stoichiometrically. C is atleast one of F, Cl, N, and S, including combinations, substituting foroxygen in amounts ranging from about 0.1 to about 100 percentstoichiometrically.

In an alternative embodiment, the present green-emitting, garnet-basedphosphors may be described by the formula(Y_(1-x)Ba_(x))₃Al₅(O_(1-y)C_(y))₁₂:Ce³⁺, where x and y each range fromabout 0.001 to about 0.2. In a variation of this embodiment, thegarnet-based phosphors may be represented by the formula(Y_(1-x)Ba_(x))_(z)Al₅(O_(1-y)C_(y))_(12+(3/2)z):Ce³⁺, where z is notequal to 3 in this embodiment, and ranges from about 2.5 to about 3.5.In these embodiments, when the constituent elements are yttrium, barium,aluminum, oxygen, and fluorine.

The present green-emitting, garnet based phosphors may be excited byblue light emitted by either a laser or LED (or any other such means),and used in combination with either of (or both) a yellow-green-emittingsilicate phosphor and/or a red-emitting nitride-based phosphor. The rednitride may have the general formula (Ca,Sr)AlSiN₃: Eu²⁺, furthercomprising an optional halogen, and wherein the oxygen impurity contentof the red nitride phosphor may be less than equal to about 2 percent byweight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the SEM morphology of Lu_(2.91)Ce_(0.09)Al₅O₁₂ withdifferent MgF₂ additive concentrations, illustrating that particle sizesbecome larger and more homogeneous as the amount of the MgF₂ additive isincreased;

FIG. 2 is a series of x-ray diffraction (XRD) patterns of exemplaryY_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations;

FIG. 3 is a series x-ray diffraction (XRD) patterns of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations;

FIG. 4 is a series of the x-ray diffraction (XRD) patterns of ofexemplary Lu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors having a 5 wt % MgF₂additive and a 5 wt % SrF₂ additive;

FIG. 5 is the emission spectra of a series of exemplaryY_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different levels of MgF₂additive, the emission spectra obtained by exciting the phosphors with ablue LED;

FIG. 6 is the normalized emission spectra of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations under blue LED excitation;

FIG. 7 is the emission spectra of Lu_(2.91)Ce_(0.09)Al₅O₁₂ phosphorswith different MgF₂ additive under blue LED excitation;

FIG. 8 is the normalized emission spectra of Lu_(2.91)Ce_(0.09)Al₅O₁₂phosphors with different MgF₂ additive under blue LED excitation; theresults show that the emission peak of Lu_(2.91)Ce_(0.09)Al₅O₁₂ shiftsto short wavelength with certain amount of MgF₂ additive, and that thegreater the amount of the MgF₂ additive, the shorter emission peakwavelength;

FIG. 9 is a normalized emission spectra of a Lu_(2.91)Ce_(0.09)Al₅O₁₂phosphor with 5 wt % MgF₂ and 5 wt % SrF₂ additives where the phosphorhas been excited with a blue LED; the results are compared with acontrol sample that contains no halogenated salts as an additive; theresults illustrate that the emission peak shifts to shorter wavelengthswith the MgF₂ synthesized compound than it does for the SrF₂ synthesizedcompound;

FIG. 10 shows how the emission wavelength of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors decreases as the concentration of anSrF₂ additive is increased;

FIG. 11 is the normalized excitation spectra of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations, showing that the excitation spectra becomes more narrowwhen the MgF₂ additive concentration is increased;

FIG. 12 shows the temperature dependence of an exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphor with a 5 wt % MgF₂ additive;

FIG. 13 shows the spectra of a white LED that includes an exemplarygreen-emitting, garnet-based phosphor having the formulaLu_(2.91)Ce_(0.09)Al₅O₁₂ with 5 wt % SrF₂ additive; the white LED alsoincludes a red phosphor having the formula (Ca_(0.2)Sr_(0.8))AlSiN₃:Eu²⁺, and when both green and red phosphors are excited with an InGaNLED emitting blue light, the resulting white light had the colorproperties CIE x=0.24, and CIE y=0.20;

FIG. 14 is the spectra of a white LED with the following components: ablue InGaN LED, a green garnet having the formulaLu_(2.91)Ce_(0.09)Al₅O₁₂ with either 3 or 5 wt % additives, a rednitride having the formula (Ca_(0.2)Sr_(0.8))AlSiN₃: Eu²⁺ or a silicatehaving the formula (Sr_(0.5)Ba_(0.5))₂SiO₄: Eu²⁺, wherein the whitelight has the color coordinates CIE (x=0.3, y=0.3); and

FIG. 15 is the spectra of the white LED systems of FIG. 14, in thisinstance measured at 3,000 K.

DETAILED DESCRIPTION OF THE INVENTION

A yttrium aluminum garnet compound activated with the rare earth cerium(YAG:Ce) is one of the best choices of phosphor material one can make ifthe desired application is either high power LED lighting, or cool whitelighting of a non-specific, general nature. As one might expect there,is a requirement in general lighting for highly efficient components,both in the case of the LED chip supplying blue light and excitationradiation, and in the case of the phosphor that is used in conjunctionwith the chip, excited by the chip, used in conjunction with the chip,and which supplies the typically yellow/green constituent of theresulting product light.

As discussed in the previous section of this disclosure, YAG:Ce doesdemonstrate this desired high efficiency, having a quantum efficiencygreater than about 95 percent, and it would therefore appear to be adifficult task to improve upon this number. But it is known in the artthat the efficiency of an LED chip increases with a decrease in emissionwavelength, and thus it would appear, in theory anyway, that theefficiency of a general lighting system will be enhanced if a phosphorpaired with an LED chip emitting at shorter wavelengths may be excitedby those shorter wavelengths. The problem with this strategy,unfortunately, is that the emission efficiency of a YAG:Ce phosphordecreases when the wavelength of its blue excitation radiation isreduced to a level below about 460 min.

The repercussions of this are, of course, that YAG:Ce should really onlybe paired with an LED chip having an emission wavelength no less thanabout 450 to 460 nm. But it is also known in the art that photonenergies of the phosphor's excitation radiation depend strongly on thestructure of the anionic polyhedron (comprising oxygen atoms in thiscase) surrounding the activator cation (cerium). It follows that theefficiency of the system may be enhanced if the excitation range of agarnet-based phosphor might be extended towards shorter wavelengthsrelative to a YAG:Ce phosphor. Thus the objects of the present inventioninclude altering the structure and nature of this anionic polyhedron toshift the excitation range the phosphor “desires” to see to shorterwavelengths than that of the traditional YAG:Ce, while maintaining inthe meantime (or even improving) the superior properties that garnetsdisplay.

The present disclosure will be divided into the following sections:first, a chemical description (stoichiometric formulas) of the presentgreen garnets will be given, followed by a brief description of possiblesynthetic methods. The structure of the present green garnets will bediscussed next, along with its relationship to experimental datacomprising wavelength and photoluminescent changes upon the inclusion ofcertain halogen dopants. Finally, the role these green garnets may playin white light illumination and backlighting applications will bepresented with exemplary data.

Chemical Description of the Present Green Garnets

The green emitting garnets of the present invention contain bothalkaline earth and halogen constituents. These dopants are used toachieve the desired photoemission intensity and spectral properties, butthe fact that simultaneous alkaline earth and halogen substitutionsprovide a sort of self-contained charge balance is fortuitous as well.Additionally, there may be other advantageous compensations having to dowith the overall changes to the size of the unit cell: while substationsof Lu for Y may tend to expand the size of the cell, the opposite effectmay occur with substitutions of an alkaline earth for Y (in some cases,at any rate), and likewise with the halogen for oxygen.

There are several ways to describe the formula of the present phosphors.In one embodiment, a yellow to green emitting cerium-doped, garnet-basedphosphor may be described by the formula(Lu_(1-a-b-c)Y_(a)Tb_(b)A_(c))₃(Al_(1-d)B_(d))₅(O_(1-e)C_(e))₁₂:Ce,Eu,where A is selected from the group consisting of Mg, Sr, Ca, and Ba; Bis selected from the group consisting of Ga and In; C is selected fromthe group consisting of F, Cl, and Br; 0≦a≦1; 0≦b≦1; O<c≦0.5; 0≦d≦1; and0<e≦0.2. The “A” element, which may be any of the alkaline earthelements Mg, Sr, Ca, and Ba, used either solely or in combination, isvery effective in shifting wavelength to shorter values. These compoundswill be referred to in the present disclosure as “halogenated LAG-based”garnets.

In an alternative embodiment, the present green garnets may berepresented by the formula (Y,Lu,A)_(x)(Al)₅(O,F,Cl)_(12+(3/2)x);subject to the proviso that x is not equal to 3, and ranges from about2.5 to about 3.5. As in the first formula described in this section, Ais selected from the group consisting of Mg, Sr, Ca, and Ba, and rangesin content stoichiometrically from greater than equal to zero to about0.5, relative to the total amounts of yttrium and lutetium. Yttrium andlutetium are interchangeable with one another. These compounds may becollectively described in the present disclosure as “non-integerstoichiometric compounds” based on YAG and LAG.

In an alternative embodiment, the present green-emitting, garnet-basedphosphors may be described by the formula (Y,A)₃(Al,B)₅(O,C)₁₂:Ce³⁺,where A is at least one of Tb, Gd, Sm, La, Lu, Sr, Ca, and Mg, includingcombinations of those elements, wherein the amount of substitution ofthose elements for Y ranges from about 0.1 to about 100 percent in astoichiometric manner. B is at least one of Si, Ge, B, P, and Ga,including combinations, and these elements substitute for AI in amountsranging from about 0.1 to about 100 percent stoichiometrically. C is atleast one of F, Cl, N, and S, including combinations, substituting foroxygen in amounts ranging from about 0.1 to about 100 percentstoichiometrically.

In an alternative embodiment, the present green-emitting, garnet-basedphosphors may be described by the formula(Y_(1-x)Ba_(x))₃Al₅(O_(1-y)C_(y))₁₂:Ce³⁺, where x and y each range fromabout 0.001 to about 0.2. In a variation of this embodiment, thegarnet-based phosphors may be represented by the formula(Y_(1-x)Ba_(x))_(z)Al₅(O_(1-y)C_(y))_(12+(3/2)z):Ce³⁺, where z is notequal to 3 in this embodiment, and ranges from about 2.5 to about 3.5.In these embodiments, when the constituent elements are yttrium, barium,aluminum, oxygen, and fluorine, the phosphor is excitable by radiationranging in wavelength from about 440 to about 470 nm, and exhibits apeak emission wavelength as a result that ranges from about 540 to about560 nm.

Synthesis

Any number of methods may be used to synthesize the presentgreen-emitting, garnet-based phosphors, involving both solid statereaction mechanisms, as well as liquid mixing techniques. Liquid mixingincludes such methods as co-precipitation and sol-gel techniques.

One embodiment of preparation involves a solid state reaction mechanismcomprising the steps:

-   -   (a) desired amounts of the starting materials CeO₂, Y₂O₃,        lutetium salts including the nitrates, carbonates, halides,        and/or oxides of lutetium, and M²⁺X₂, where M is a divalent        alkaline earth metal selected from the group consisting of Mg,        Sr, Ca, and Ba, and X is a halogen selected from the group        consisting of F and Cl were combined to form a mixture of        starting powders;    -   (b) the mix of starting powders from step (a) is dry-mixed using        any conventional method, such as ball milling, and typical        mixing times using ball milling are greater than about 2 hours        (in one embodiment about 8 hours);    -   (c) sintering the mixed starting powders from step (b) at a        temperature of about 1400° C. to about 1600° C. for about 6 to        about 12 hours in a reducing atmosphere (the purpose of this        atmosphere is for a reduction of the ammonia-based compounds);    -   (d) crushing the sintered product from step (c), and washing it        with water; and    -   (e) drying the washed product from step (d), wherein the drying        conditions may be constitute a time of about 12 hours at a        temperature of about 150° C.

The present garnets may be synthesized by liquid mixing techniques aswell. An example of the synthesis of a non-halogenated LAG compoundhaving the formula Lu_(2.985)Ce_(0.015)Al₅O₁₂ using co-precipitation hasbeen described by H.-L. Li et al. in an article titled “Fabrication ofTransparent Cerium-Doped Lutetium Aluminum Garnet Ceramics byCo-Precipitation Routes,” J. Am. Ceram. Soc. 89 [7] 2356-2358 (2006).These non-halogenated LAG compounds contained no alkaline earthconstituents. The article is incorporated herein in its entirety, as itis contemplated that a similar co-precipitation method may be used toproduce the halogenated LAGs of the present disclosure with alkalineearth constituents.

An example of the synthesis of a halogenated YAG compound using asol-gel technique has been described in U.S. Pat. No. 6,013,199 by E.McFarland et al., to Syrnyx Technologies, titled “Phosphor materials.”These (possibly) halogenated YAG compounds contained no alkaline earthconstituents. This patent is incorporated herein in its entirety, as itis contemplated that a similar sol-gel method may be used to produce thehalogenated YAG compounds of the present disclosure with alkaline earthconstituents.

FIG. 1 shows the SEM morphology of an exemplary Lu_(2.91)Ce_(0.09)Al₅O₁₂phosphors with different MgF₂ additive concentrations, synthesized viathe solid state mechanisms described above. The morphology as revealedby scanning electron microscope (SEM) shows that particle sizes becomelarger, and more homogeneous, as the amount of the MgF₂ additive isincreased.

Crystal Structure of the Present Green Garnets

The crystal structure of the present green garnets are the same as thatof the yttrium aluminum garnet, Y₃Al₅O₁₂, and like this well studied YAGcompound, the present garnets belong to the space group Ia3d (no. 230).This space group, as it pertains to YAG, has been discussed by Y. Kuruet al. in an article titled “Yttrium Aluminum Garnet as a Scavenger forCa and Si,” J. Am. Ceram. Soc. 91 [11] 3663-3667 (2008). As described byY. Kuru et al., YAG has a complex crystal consisting of 160 atoms (8formula units) per unit cell, where the Y³⁺ occupy positions ofmultiplicity 24, Wyckoff letter “c,” and site symmetry 2.22, and the 0²⁻atoms occupy positions of multiplicity 96, Wyckoff letter “h,” and sitesymmetry 1. Two of the Al³⁺ ions are situated on octahedral 16(a)positions, whereas the remaining three Al³⁺ ions are positioned ontetrahedral 24(d) sites.

The lattice parameters of the YAG unit cell are a=b=c=1.2008 nm, anda=β=γ=90°. Whereas substitution of lutetium for yttrium is expected toexpand the size of the unit cell, the angles between the unit cell axesare not expected to change, and the material will retain its cubiccharacter.

FIG. 2 shows the x-ray diffraction (XRD) patterns of a series ofexemplary Y_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations, showing how the addition of an alkaline earth and ahalogen (MgF₂) component shifts high angle diffraction peaks to highervalues of 2θ. This means that the lattice constants become smallerrelative to a YAG component with no alkaline earth/halogen, and furtherindicates that Mg²⁺ is being incorporated into the crystal lattice,occupying Y³⁺ positions.

FIG. 3 shows the x-ray diffraction (XRD) pattern of a series ofexemplary phosphors in an analogous manner to FIG. 2, except that thistime the series of compounds are Lu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors withdifferent MgF₂ additive concentrations, where lutetium-based compoundsare being studied, rather than yttrium-based compounds.

FIG. 4 shows the x-ray diffraction (XRD) pattern of a series ofexemplary Lu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors having either a 5 wt % MgF₂and 5 wt % SrF₂ additive: this experiment shows a comparison of the Mgconstituent versus a Sr one. The data shows that with the MgF₂ additivein the Lu_(2.91)Ce_(0.09)Al₅O₁₂ lattice, high angle diffraction peakmove to greater values of 2θ, meaning that lattice constants becomesmaller. Alternatively, with SrF₂ additive, high angle diffraction peaksmove to smaller values of 2θ, meaning that the lattice constantsincrease. It will be apparent to those skilled in the art that both Mg²⁺and Sr²⁺ are being incorporated into the Lu_(2.91)Ce_(0.09)Al₅O₁₂lattice and occupying Lu³⁺ positions. These peak shifts in positionoccur because Mg²⁺, with its ionic radius of 0.72 Å, is smaller thanLu³⁺ (0.86 Å), while Sr²⁺ (1.18 Å) is bigger than Lu³⁺.

Mechanism of Alkaline Earth and Halogen Influence on Optical Properties

In one embodiment of the present invention, Ce³⁺ is the luminescentactivator in the garnet-based phosphor. The transition between the 4fand 5d energy levels of the Ce³⁺ ion corresponds to excitation of thephosphor with blue light; green light emission from the phosphor is aresult from the same electronic transition. In the garnet structure, theCe³⁺ is located at the center of an octahedral site formed by apolyanionic structure of six oxygen ions. It will be appreciated bythose skilled in the art that according to crystal field theory, thesurrounding anions (which may also be described as ligands) induce anelectrostatic potential on the 5d electron of the central cation. The 5denergy level splitting is 10Dq, where Dq is known to depend on theparticular ligand species. From the spectrochemical series it may beseen that the Dq of a halide is smaller than that of oxygen, and thus itfollows that when oxygen ions are replaced by halide ions, the Dq willdecrease correspondingly.

The implications of this are that the band gap energy; that is to say,the energy difference between the 4f and 5d electronic levels, willincrease with substitution of oxygen ions with halide in the polyanioniccages surrounding activator ions. This is why the emission peak shiftsto shorter wavelength with halogen substitution. Simultaneously, withthe introduction of halide ions in the oxygen polyanionic structuresforming octahedral sites, a corresponding cation may also replace aportion of the Lu/Y content. If the cation replacing Lu/Y is a smallercation, the result will be a shift of the emission peak towards the blueend of the spectrum. The emitted luminescence will have a shorterwavelength than otherwise would have occurred. In contrast, if thecation replacing Lu/Y is a larger cation, such as Sr or Ba, the resultwill be a shift of the emission peak towards the red end of thespectrum. In this case, the emitted luminescence will have a longerwavelength.

Combined with the effects of the halide, Mg as an alkaline earthsubstituent will be a better choice than Sr if a blue-shift is desired,and this will be shown experimentally in the following portions of thepresent disclosure. It is also known the LAG emission peak is a doubletdue to spin-orbit coupling. As the blue-shift occurs, the emission withshorter wavelength is biased and its intensity increasescorrespondingly. This trend is not only helpful to the blue-shift of theemission, but also enhances photoluminescence.

FIG. 5 is the emission spectra of a series of exemplaryY_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different levels of MgF₂additive, the emission spectra obtained by exciting the phosphors with ablue LED. This data shows that with increasing amounts of MgF₂ thephotoluminescent intensity increases and the peak emission wavelengthshifts to shorter values. Though not shown on FIG. 5, the presentinventors have data for a 5 wt % addition of BaF₂ to the startingpowders: this phosphor showed a significant increase in photoluminescentintensity relative to the three magnesium-containing phosphors, and apeak emission wavelength that is about the same as that of the 1 wt %sample.

A normalized version of the data from FIG. 5 is shown in FIG. 6. FIG. 6is the normalized emission spectra of the same series of exemplaryY_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations under blue LED excitation, but where normalizing thephotoluminescent intensity to a single value highlight that the emissionpeak of Y_(2.91)Ce_(0.09)Al₅O₁₂ shifts to short wavelength withincreasing amounts of the MgF₂ additive. The greater the amount of theMgF₂ additive, the shorter the emission peak wavelength. This is thesame trend demonstrated by a Lu_(2.91)Ce_(0.09)Al₅O₁₂ phosphor, as willbe demonstrated next.

FIG. 7 is the emission spectra of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different levels of MgF₂additive, the emission spectra obtained by exciting the phosphor with ablue LED. This data is analogous to that of FIG. 5, except thatlutetium-based rather than yttrium-based compounds are being studied. Aswith the yttrium data, this data for lutetium shows similar trends forthe shift in emission wavelength, though those trends forphotoluminescent intensity are not, perhaps, as similar.

The Lu_(2.91)Ce_(0.09)Al₅O₁₂ emission spectra of FIG. 7 have beennormalized to emphasize the effect of the addition of halogen salt onpeak emission wavelength; the normalized version of the data is shown inFIG. 8. As in the yttrium case, peak emission shifts to shorterwavelength with increasing amounts of MgF₂ additive; that is to say, thegreater the amount of the MgF₂ additive, the shorter the emission peakwavelength. The amount of wavelength shift upon increasing the amount ofthe MgF₂ additive from zero (no additive) to about 5 wt % of theadditive was observed to be about 40 nm; from about 550 nm to about 510nm.

Each of the graphs in FIGS. 5-8 have plotted their respective spectra asa series of phosphor compositions with increasing additiveconcentration, starting at no additive, and ending with the highestconcentration of the series at 5 wt %. To emphasize a comparison of theSrF₂ additive with the MgF₂ additive; in other, words, a phosphor with aSr alkaline earth and fluorine content with a phosphor having a Mgalkaline earth and fluorine content, the phosphors have been plottedtogether in FIG. 9: a phosphor with no additive, a phosphor with 5 wt %SrF₂, and a phosphor with 5 wt % MgF₂. The phosphor is based on thesample Lu_(2.91)Ce_(0.09)Al₅O₁₂.

The emission spectra data in FIG. 9 has been normalized to betteremphasize the effects on optical properties rendered by the inclusion ofthe halogen and alkaline earths. When excited with a blue LED, theresult illustrates that the emission peak shifts to shorter wavelengthswith the addition of MgF₂ and SrF₂. The Lu_(2.91)Ce_(0.09)Al₅O₁₂ samplewith no additive shows a peak emission wavelength at about 550 nm; witha 5 wt % SrF₂ additive the peak emission wavelength shifts to about 535nm, and with a 5 wt % MgF₂ additive the wavelength shifts even furtherto about 510 nm.

FIG. 10 shows how the emission wavelength of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors decreases as the concentration of aSrF₂ additive is increased. Peak emission wavelength has been plotted asa function of the amount of the SrF₂ additive; samples having a SrF₂additive content of 1, 2, 3, and 5 wt % were tested. The results showthat the peak emission wavelength was about the same for the 1 and 2 wt% samples, the wavelength being about 535 nm; as the SrF₂ additive isincreased to 3 wt % the peak emission wavelength decreases to about 533nm. With a further increase of SrF₂ additive to 5 wt % peak wavelengthdrops precipitously to about 524 nm.

Excitation Spectra and Temperature Dependence

FIG. 11 is the normalized excitation spectra of a series of exemplaryLu_(2.91)Ce_(0.09)Al₅O₁₂ phosphors with different MgF₂ additiveconcentrations, showing that the excitation spectra become more narrowwhen the MgF₂ additive concentration is increased. The data shows thatthe present green garnets exhibit a wide band of wavelengths over whichthe phosphors may be excited, ranging from about 380 to about 480 nm.

The thermal stability of the present garnet phosphors is exemplified bythe lutetium containing compound Lu_(2.91)Ce_(0.09)Al₅O₁₂ with a 5 wt %MgF₂ additive; in FIG. 12, its thermal stability is compared with thecommercially available phosphor Ce³⁺:Y₃Al₅O₁₂. It may be observed thatthe thermal stability of the Lu_(2.91)Ce_(0.09)Al₅O₁₂ compound is evenbetter than the YAG.

Applications to Backlighting and White Light Illumination Systems

According to further embodiments of the present invention, the presentgreen garnets may be used in white light illumination systems, commonlyknown as “white LEDs,” and in backlighting configurations for displayapplications. Such white light illumination systems comprise a radiationsource configured to emit radiation having a wavelength greater thanabout 280 nm; and a halide anion-doped green garnet phosphor configuredto absorb at least a portion of the radiation from the radiation source,and emit light having a peak wavelength ranging from 480 mm to about 650nm.

FIG. 13 shows the spectra of a white LED that includes an exemplarygreen-emitting, garnet-based phosphor having the formulaLu_(2.91)Ce_(0.09)Al₅O₁₂ with a 5 wt % SrF₂ additive. This white LEDfurther includes a red phosphor having the formula(Ca_(0.2)Sr_(0.8))AlSiN₃: Eu²⁺. When both green garnet and red nitridephosphors are excited with an InGaN LED emitting blue light, theresulting white light displayed the color coordinates CIE x=0.24, andCIE y=0.20. The sample in FIG. 13 that contains the yellow-greensilicate

FIG. 14 is the spectra of a white LED with the following components: ablue InGaN LED, a green garnet having the formulaLu_(2.91)Ce_(0.09)Al₅O₁₂ with either 3 or 5 wt % additives, a rednitride having the formula (Ca_(0.2)Sr_(0.8))AlSiN₃: Eu²⁺ or a silicatehaving the formula (Sr_(0.5)Ba_(0.5))₂SiO₄: Eu²⁺, wherein the whitelight has the color coordinates CIE (x=0.3, y=0.3). The sample thatshows the most prominent double peak is the one labeled “EG3261+R640,”where the EG3261 designation represents the (Sr_(0.5)Ba_(0.5))₂SiO₄:Eu²⁺ phosphor, in combination with the red R640(Ca_(0.2)Sr_(0.8))AlSiN₃: Eu²⁺ phosphor emitting at about 640 nm. Thetwo peaks labeled LAG (3 wt % MgF₂)+R640 and LAG (5 wt % SrF₂)+R640demonstrate a much more uniform emission of perceived white light overthe wavelength range 500 to 650 nm, an attribute desirable in the art.

FIG. 15 is the spectra of the white LED systems of FIG. 14, in thisinstance measured at 3,000 K.

In embodiments of the present invention, the red nitride that may beused in conjunction with the green garnet may have the general formula(Ca,Sr)AlSiN₃:Eu²⁺, where the red nitride may further comprise anoptional halogen, and wherein the oxygen impurity content of the rednitride phosphor may be less than or equal to about 2 percent by weight.

Optical and Physical Data in Table Form

A summary of exemplary data is tabulated in the following two tables. InTable 1 is the testing results of a Lu_(2.91)Ce_(0.09)Al₅O₁₂ basedphosphor with three different MgF₂ additive levels. Table 2 tabulatesthe testing results of a Lu_(2.91)Ce_(0.09)Al₅O₁₂ based compound withfour different SrF₂ additive levels. These results summarize and confirmthat MgF₂ and SrF₂ additives in Lu_(2.91)Ce_(0.09)Al₅O₁₂ shift theemission peak wavelength to shorter wavelengths, where the emissionintensity is increased with increasing MgF₂ and SrF₂ concentration. Theparticle size also increases with the increasing MgF₂ and SrF₂ additiveconcentration.

TABLE 1 Testing results of Lu_(2.91)Ce_(0.09)Al₅O₁₂ with different MgF₂levels of additive Emission Peak MgF₂ Wavelength Relative Particle Size(wt %) CIE x CIE y (nm) Intensity (%) D50 (μm) 1 0.3635 0.5862 526.8858.04 4.01 2 0.3554 0.5778 529.56 78.47 7.38 3 0.3336 0.5776 514.22105.13 9.30

TABLE 2 Testing results of Lu_(2.91)Ce_(0.09)Al₅O₁₂ with differentlevels of SrF₂ additive Emission Peak SrF₂ Wavelength Relative ParticleSize (wt %) CIE x CIE y (nm) Intensity (%) D50 (μm) 1 0.3677 0.5732534.64 71.65 3.84 2 0.3677 0.5732 534.64 85.82 5.24 3 0.3555 05718532.43 112.40 9.90 5 0.3405 0.5748 523.44 107.67 11.38

What is claimed is:
 1. A cerium-activated, green-emitting lutetiumaluminate phosphor comprising lutetium, cerium aluminum, oxygen,fluorine, and strontium, wherein said phosphor is configured to absorbexcitation radiation having a wavelength ranging from about 380 nm toabout 480 nm, and to emit light having a peak emission wavelengthranging from about 523.44 nm to about 534.64 nm, wherein said phosphoris characterized by CIE(x) coordinates ranging from 0.3405 to 0.3677 andCIE(y) coordinates ranging from 0.5718 to 0.5748.
 2. Thecerium-activated, green-emitting lutetium aluminate phosphor of claim 1,wherein phosphor particle size is characterized by a particle sizedistribution with D50 ranging from about 3.84 microns to 11.38 microns.3. A cerium-activated, green-emitting lutetium aluminate phosphorcomprising lutetium, cerium, aluminum, oxygen, fluorine, and magnesium,wherein said phosphor is configured to absorb excitation radiationhaving a wavelength ranging from about 380 nm to about 480 nm, and toemit light having a peak emission wavelength ranging from about 514.22nm to about 529.56 nm, wherein said phosphor is characterized by CIE(x)coordinates ranging from 0.3336 to 0.3635 and CIE(y) coordinates rangingfrom 0.5776 to 0.5862.
 4. The cerium-activated, green-emitting lutetiumaluminate phosphor of claim 3, wherein phosphor particle size ischaracterized by a particle size distribution with D50 ranging fromabout 4.01 microns to 9.30 microns.
 5. The cerium-activated,green-emitting lutetium aluminate phosphor of claim 3, wherein saidexcitation radiation has a wavelength ranging from about 420 nm to about480 nm.
 6. A white LED comprising: a radiation source configured toprovide radiation having a wavelength greater than about 280 nm; acerium-activated, green-emitting lutetium aluminate phosphor comprisinglutetium, cerium, aluminum, oxygen, fluorine, and strontium, whereinsaid phosphor is configured to absorb excitation radiation having awavelength ranging from about 380 nm to about 480 nm, and to emit lighthaving a peak emission wavelength ranging from about 523.44 nm to about534.64 nm, wherein said phosphor is characterized by CIE(x) coordinatesranging from 0.3405 to 0.3677 and CIE(y) coordinates ranging from 0.5718to 0.5748; and at least one of a red-emitting phosphor or ayellow-emitting phosphor.
 7. The white LED of claim 6, wherein thered-emitting phosphor is a nitride.
 8. The white LED of claim 7, whereinthe nitride has the formula (Ca,Sr)AlSiN₃:Eu²⁺.
 9. The white LED ofclaim 6, wherein the yellow-emitting phosphor is a silicate.
 10. Thewhite LED of claim 9, wherein the silicate has the formula(Sr,Ba)₂SiO₄:Eu²⁺.
 11. The cerium-activated, green-emitting lutetiumaluminate phosphor of claim 1, wherein said excitation radiation has awavelength ranging from about 420 nm to about 480 nm.
 12. A white LEDcomprising: a radiation source configured to provide radiation having awavelength greater than about 280 nm; a cerium-activated, green-emittinglutetium aluminate phosphor comprising lutetium, cerium, aluminum,oxygen, fluorine, and magnesium, wherein said phosphor is configured toabsorb excitation radiation having a wavelength ranging from about 380nm to about 480 nm, and to emit light having a peak emission wavelengthranging from about 514.22 nm to about 529.56 nm, wherein said phosphoris characterized by CIE(x) coordinates ranging from 0.3336 to 0.3635 andCIE(y) coordinates ranging from 0.5776 to 0.5862; and at least one of ared-emitting phosphor or a yellow-emitting phosphor.
 13. The white LEDof claim 12, wherein the red-emitting phosphor is a nitride.
 14. Thewhite LED of claim 13, wherein the nitride has the formula(Ca,Sr)AlSiN₃:Eu²⁺.
 15. The white LED of claim 12, wherein theyellow-emitting phosphor is a silicate.
 16. The white LED of claim 15,wherein the silicate has the formula (Sr,Ba)₂SiO₄:Eu²⁺.